Migration and plasma enhanced chemical vapor deposition

ABSTRACT

A method of producing a thin film using plasma enhanced chemical vapor deposition, including the steps of supplying a cation species to a substrate region when there is at most a relatively low flux of a plasma based anion species in the substrate region, and supplying the plasma based anion species to the substrate region when there is at most a relatively low flux of the cation species in the substrate region. This enables delivery of gaseous reactants to be separated in time in PECVD and/or RPECVD based film growth systems, which provides a significant reduction in the formation of dust particles for these plasma based film growth techniques.

CROSS-REFERENCE

This application claims the benefit of Canadian Patent Application No.2653581, entitled “Migration and plasma enhanced chemical vapourdeposition”, filed on Feb. 11, 2009, and U.S. Provisional PatentApplication No. 61/185,953, entitled “Migration and plasma enhancedchemical vapour deposition”, filed on Jun. 10, 2009, which are entirelyincorporated herein by reference.

TECHNICAL FIELD

The present invention generally relates to the field of thin films, andmore specifically to a method of and/or device for thin film crystalgrowth using plasma enhanced chemical vapor deposition, which may beremote plasma enhanced chemical vapor deposition.

BACKGROUND OF THE INVENTION

Presently, inadvertent dust formation is a major problem facing manyknown techniques of thin film crystal growth that involve a gas or vaporphase reactant, including chemical vapor deposition (CVD) methods, suchas metalorganic chemical vapor deposition (MOCVD), plasma enhancedchemical vapor deposition (PECVD), and remote plasma enhanced chemicalvapor deposition (RPECVD), and physical forms of thin film depositionsuch as reactive sputtering and reactive evaporation.

Similarly, HVPE (sometimes called hydride vapor phase epitaxy or halidevapor phase epitaxy) can be affected by dust formation. Dust formationoccurs when reactions take place in the gas phase between the reactingspecies, instead of on the surfaces of the “substrates” on which thefilms are intentionally grown. Molecules formed in the gas phasecoalesce into larger dust particles. Dust formation is often the resultof parasitic gas phase reactions and is an unwanted effect during thinfilm growth. Dust formation can cause dust to build up on the walls of areaction chamber so that the reactor chamber eventually requires downtime for cleaning. In severe cases the dust can be incorporated in thefilm itself, or no film may grow because dust formation is entirelyprevalent.

There are some known techniques to attempt to reduce the formation ofsuch parasitic dust by suppressing the gas phase reactions that lead todust formation. For example by chemical means, although chemicaltechniques that suppress parasitic gas phase reactions can have theunwanted effect of also suppressing the reactions that cause film growthon a substrate. Another means of attempting to suppress dust formationis to reduce the density of the reacting species in the gas phase of afilm growth region. This is done so that the individual molecules orother reacting species are less likely to collide with one and other andundergo a chemical interaction before reaching the substrate, thoughthis can have the unwanted effect of lowering the film growth rate. Itmay also be possible to lower the temperature of the gas so thatparasitic reactions in the gas phase are less prevalent, but this is notalways easily achieved above a heated substrate, or with plasma heatingpresent.

For the film growth techniques of PECVD and RPECVD, in particular, ithas been observed by the inventor that the high delivery rates forreactant species that would otherwise lead to high growth rates is notpossible due to severe dust formation. Hence these known techniques arelimited in terms of their potential film growth rates.

There is thus a need for a method and/or device that addresses or atleast ameliorates one or more problems inherent in current systems andmethods.

The reference in this specification to any prior publication (orinformation derived from the prior publication), or to any matter whichis known, is not, and should not be taken as an acknowledgment oradmission or any form of suggestion that the prior publication (orinformation derived from the prior publication) or known matter formspart of the common general knowledge in the field of endeavor to whichthis specification relates.

SUMMARY OF THE INVENTION

According to one aspect, the present invention seeks to reduce theformation of dust during thin film growth by relatively high pressurethin film growth techniques, for example, operating approximately over arange of 0.1 mTorr to 10 Torr.

Preferably, delivery of gaseous reactants is separated in time in plasmaenhanced chemical vapor deposition (PECVD) and/or remote plasma enhancedchemical vapor deposition (RPECVD) based film growth systems, whichprovides a significant reduction in the formation of dust particles forthese plasma based film growth techniques.

According to a first example form, there is provided a method ofproducing a thin film using plasma enhanced chemical vapor deposition(reference to which includes both PECVD and RPECVD), including the stepsof: supplying a cation species to a substrate region when there is atmost a relatively low flux of a plasma based anion species in thesubstrate region; and, supplying the plasma based anion species to thesubstrate region when there is at most a relatively low flux of thecation species in the substrate region.

According to a second example form, there is provided a method of thinfilm crystal growth using plasma enhanced chemical vapor deposition,including the step of intermittently modulating the supply of a cationspecies and an anion species to a substrate region.

In another aspect of the invention, a method for forming a Group IIImetal nitride thin film over a substrate in a reaction chamber isprovided, comprising alternately and sequentially pulsing into thereaction chamber a Group III metal precursor and plasma-activatedspecies of nitrogen, each pulse of the Group III metal precursor forminga non-self limiting layer of a Group III metal over the substrate.

Reference to supply of a cation species or an anion species can be readas also referring to supply of a cation species precursor or an anionspecies precursor. In an embodiment, cation species can include GroupIII metal precursors, such as, e.g., metalorganic species (also“organometallic species” and “metal organic species” herein). In anembodiment, anion species can include active neutral species of N₂.

Reference to modulating the supply of the cation species or the anionspecies to a substrate region can be read as encompassing any means ofachieving such an effect. For example, modulating the supply of aspecies could be achieved by: a pressure of a species could be modifiedat or remote to the substrate region; a flow rate of injecting a speciesinto a chamber could be modified; an evaporation rate of a species couldbe modified; a physical, electric or magnetic barrier could be used tomodulate flux of a species between distinct areas; a pressure of abackground gas, if present, could be modified; a plasma excitationsource could be modified such as pulsed on or off; combinations of theforegoing; and/or various other mechanisms.

Reference to intermittently modulating the supply of the species can beread as any form of intermittent, periodic, interspersed, pulsed, or thelike, modulation of two or more species. In a preferred example,modulation of the supply of each species is out of phase so that amaximum rate of supply of a first species is intermittent to a maximumrate of supply of a second species. The period, frequency and amplitudefor modulation of each species can be independently changed as desired.

INCORPORATION BY REFERENCE

All publications (e.g., scientific publications, engineeringpublications), patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, or patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1 illustrates a method for producing a thin film using plasmaenhanced chemical vapor deposition, in accordance with an embodiment ofthe invention;

FIG. 2 is a pulsing diagram showing pulses of a metalorganic species andactive neutral species of nitrogen (“active nitrogen”), in accordancewith an embodiment of the invention;

FIG. 3 illustrates a schematic of an example plasma processing reactor,in accordance with an embodiment of the invention;

FIG. 4 illustrates a schematic of an example main chamber of a plasmaprocessing reactor, in accordance with an embodiment of the invention;

FIG. 5 illustrates a schematic of an example shower head (also“showerhead” herein) feed of a plasma processing reactor, in accordancewith an embodiment of the invention;

FIG. 6 is potential energy diagram for activated nitrogen, in accordancewith an embodiment of the invention;

FIG. 7A shows an image of an InN film grown with evidence of migrationenhanced epitaxy on the top layer, in accordance with an embodiment ofthe invention; and

FIG. 7B shows an image of an InN film grown with no evidence of amigration enhanced top layer, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

While embodiments of the invention are shown and described herein, itwill be obvious to those skilled in the art that such embodiments areprovided by way of example only. Numerous variations, changes, andsubstitutions will be apparent to those skilled in the art withoutdeparting from the scope of the invention. It shall be understood thatsuch alternatives to embodiments of the invention described herein areconsidered as part of the invention.

The following modes, given by way of example only, are described inorder to provide a more precise understanding of the subject matter ofpreferable embodiments of the invention.

In a preferable embodiment, plasma enhanced chemical vapor deposition(PECVD) and remote plasma enhanced chemical vapor deposition (RPECVD)based film growth systems are utilized (herein collectively referred toas plasma enhanced chemical vapor deposition, or PECVD), for example inthe growth of compound semiconductors and insulators, and the gas phasedelivery of the reactants is separated in time. This provides asignificant reduction in the formation of dust particles for theseplasma based film growth techniques. PECVD and RPECVD are generally usedat relatively low temperatures compared to thermal growth techniquessuch as metalorganic chemical vapor deposition (MOCVD), and crystalquality can be problematic at such low growth temperatures.

Methods and systems of embodiments of the invention advantageouslyreduce, if not eliminate, problems associated with dust formation andgas phase contamination during thin film formation. Methods and systemsof embodiments of the invention can be used to form improved devices.

In certain embodiments, a Group III metal precursor and active neutralspecies of N₂ are alternately and sequentially pulsed into a reactionchamber to form a Group III metal nitride film over a substrate. In anembodiment, each pulse of the Group III metal precursor forms a non-selflimiting layer of a Group III metal, which can be subsequently contactedwith active neutral species of N₂ to form a Group III metal nitride thinfilm.

“Metal nitride” can refer to a material comprising one or more metals orone or more semiconductors, and nitrogen. In certain embodiments, ametal nitride (e.g., metal nitride thin film) can have the formulaM_(x)N_(y), wherein ‘M’ designates a metal or a semiconductor, ‘N’designates nitrogen, and ‘x’ and ‘y’ are numbers greater than zero. Insome embodiments, a metal nitride can have the formula M_(z)N_(1-z),wherein ‘z’ is a number greater than zero and less than 1. In someembodiments, ‘M’ can comprise one or more metals and/or semiconductors.In embodiments, M_(x)N_(y) refers to a metal nitride, such as a GroupIII metal nitride (e.g., gallium nitride, indium nitride, aluminumgallium nitride). In some embodiments, a metal nitride film or thin filmcan comprise other materials, such as, e.g., chemical dopants. Chemicaldopants can include p-type dopants (e.g., Magnesium, Zinc) and n-typedopants (e.g., Silicon, oxygen).

“Plasma excited species” can refer to radicals, ions (cations, anions)and other excited species generated via application (or coupling) ofenergy to a reactant gas or vapor. Energy can be applied via a varietyof methods, such as, e.g., induction, ultraviolet radiation, microwavesand capacitive coupling. The plasma generator may be a direct plasmagenerator (i.e., direct plasma generation) or a remote plasma generator(i.e., remote plasma generation). In the absence of coupling energy,plasma generation is terminated. Plasma-excited species include, withoutlimitation, nitrogen radicals, nitrogen ions, and active neutral speciesof nitrogen. The source of plasma activated species may include, withoutlimitation, N₂, NH₃, and/or hydrazine. For remote plasma generation,plasma-excited species of a particular vapor phase chemical (e.g.,nitrogen containing plasma species) can be formed in a plasma generatorin fluid communication with a reaction chamber having a substrate to beprocessed.

“Adsorption” can refer to chemical attachment of atoms or molecules on asurface

“Substrate” can refer to any workpiece on which film or thin filmformation is desired. Substrates can include, without limitation,silicon, silica, sapphire, zinc oxide, SiC, AlN, GaN, Spinel, coatedsilicon, silicon on oxide, silicon carbide on oxide, glass, and indiumnitride.

“Surface” can refer to a boundary between the reaction space and afeature of the substrate.

“Cation species” can refer to a chemical, such as a vapor phasechemical, for depositing a metal or metal-containing species on or overa substrate. In embodiments, cation species can be used to deposit GroupIII metals on a substrate. A cation species can include one or moreatoms of a Group III metal desired on a substrate. In an embodiment,cation species can include one or more Group III metals selected fromboron (B), aluminum (Al), gallium (Ga) and indium (In). In variousembodiments, the cation species is a Group III metal precursor (also“Group III metal-containing reactant” herein). In certain embodiments,the Group III metal precursor is a metalorganic species. In anembodiment, the Group III metal precursor can be trimethyl gallium ortriethyl gallium. In embodiments, cation species are used to form aGroup III metal nitride thin film, M_(x)N_(y), wherein ‘M’ is a GroupIII metal, ‘N’ is nitrogen, and ‘x’ and ‘y’ are numbers greater thanzero. The cation species can provide the Group III metal (M) for formingthe metal nitride layer. In certain embodiments, ‘M’ can be a cation.

“Anion species” can refer to a chemical, such as a vapor or gas phasechemical, for providing nitrogen and/or oxygen to a metal on or over asubstrate. In embodiments, anion species can be used to provide oxygenand/or nitrogen to a Group III metal on a substrate. In otherembodiments, anion species can include mixtures of anions and noblegases, such as argon or neon, for providing oxygen and/or nitrogen to aGroup III metal on a substrate. In embodiments, anion species caninclude active neutral species of nitrogen (N₂) (also “plasma-activatedspecies of nitrogen” herein), which can be formed using a plasmagenerator.

Thin Film Growth

In an aspect of the invention, methods for forming thin films areprovided. Methods of embodiments of the invention can be used to formGroup III metal nitride thin films or layers. Group III metal thin filmsof embodiments of the invention can include one or more of boron (B),aluminum (Al), gallium (Ga), indium (In) and Thallium (Tl). In anembodiment, Group III metal thin films can comprise gallium nitride. Inanother embodiment, Group III metal thin films can comprise InN. Inanother embodiment, Group III metal thin films can comprise AlN. Inanother embodiment, Group III metal thin films can comprise alloys ofGaN, AlN, and/or InN, such as InGaN, AlGaN, and/or AlInGaN.

In embodiments, a method of thin film crystal growth using plasmaenhanced chemical vapor deposition includes intermittently modulatingthe supply of a cation species and an anion species to a substrateregion. In certain embodiments, modulating the supply of a cationspecies and an anion species comprises pulsing cation species and anionspecies to a substrate region.

In embodiments, a method of producing a thin film using plasma enhancedchemical vapor deposition is provided, comprising the steps of supplyinga cation species to a substrate region when there is at most arelatively low flux of a plasma based anion species in the substrateregion, and supplying the plasma based anion species to the substrateregion when there is at most a relatively low flux of the cation speciesin the substrate region.

Reference will now be made to the figures, wherein like numerals referto like parts throughout. It will be appreciated that the figures andstructures therein are not necessarily drawn to scale.

With reference to FIG. 1, there is illustrated a method 100 forproducing a thin film using plasma enhanced chemical vapor deposition.At step 110, a cation species is supplied to a substrate (i.e., sample)region (for thin film growth on a substrate) with no, or a relativelylow flux of a plasma based anion species being present. A relatively lowflux can depend on specific chemistry of the species, but as an examplecould be between 0% and perhaps 50% of the normal or maximum flux of aspecies when supplied for film growth. At step 120, the cations areallowed sufficient time to diffuse on the film/substrate. This can be aspart of the time duration when supplying the cations, or as a separateintermediate time delay step. In this way the cation is resident on afilm surface long enough to be able to diffuse to an energeticallyfavorable site for good film growth. This lateral growth can result ingood quality thin film material at relatively low film growthtemperatures. Then at step 130 the cation species flux is significantlyreduced or stopped and a plasma based anion species is supplied to thesubstrate region. The process can be repeated at step 140, as requiredif necessary, to obtain a desired film thickness. The modulation of thesupply of each species is preferably out of phase so that a maximum rateof supply of a first species is intermittent to a maximum rate of supplyof a second species. The period, frequency and amplitude for modulationof each species can be independently changed as desired, for example apulsed period of time for a species could vary significantly betweenabout 1 second and 30 minutes. Thus, method 100 provides a means formigration enhanced epitaxy in a plasma enhanced chemical vapordeposition system.

This novel technique is ideally suited to the growth of some compoundsemiconductors, such as group III metal nitrides, rare earth nitrides,other nitride compound species and oxide compound species. With theintroduction of migration enhanced epitaxy it is also possible to varythe growth conditions for a cation species (e.g., Group III metalsincluding Boron, Aluminum, Gallium, Indium, Thallium) and an anionspecies (e.g., Nitrogen, Oxygen) separately, which can lead to somegains in improved precursor delivery. Obviously, a wide variety of otherreactant species can be used.

In a specific but non-limiting example, the method of achievingmigration enhanced epitaxy can be applied to known RPECVD based filmgrowth of group III nitride films, for example the systems described inInternational Patent Publications WO2006/034540 and WO2003/097532, ofwhich the present inventor is a co-inventor, which are entirelyincorporated herein by reference.

An acronym that describes this technique is ME-RPECVD, or migrationenhanced RPECVD. However, RPECVD reactors can also be referred to asafterglow reactors, so that the acronym MEAGlow (migration enhancedafterglow), can be used. It should be noted that the technique can alsobe applied to PECVD systems.

In a specific illustrative example, known RPECVD based film growthmethods can be generally used for the growth of good quality galliumnitride films at growth rates of less than 150 nanometers (“nm”)/hour.The achievement of higher growth rates is desirable to lower devicedeposition time, and to thereby allow RPECVD to be more competitive withMOCVD where growth rates as high as 2-3 micrometers (“μm”)/hr can beachieved for good quality film growth.

However, achieving higher growth rates for the RPECVD growth of galliumnitride, for instance, is dependent on having a plasma source thatproduces a higher number of active nitrogen based species in the gasphase. Hence, a more efficient plasma source, capable of increasing thefilm growth rate, will incur the problem of a higher rate of dustformation. The low temperature growth by RPECVD of good crystallinequality GaN, has also been found to be less consistent than would bedesirable. By using a MEAGlow system capable of applying theaforementioned method, both these problems can be addressed.

In a particular illustrative example, a relatively short pulse of thegallium precursor material, trimethylgallium, is delivered at a muchhigher delivery rate than for normal RPECVD, which would cause theformation of excess gallium on the sample surface. The pulse is ofsufficient duration to allow diffusion of the gallium species at thesample surface. A pulse of a remote nitrogen or ammonia plasma can thenfollow the pulse of metalorganic, to supply the nitrogen species used byRPECVD for film growth. In this way the reactant species are in the gasphase at separate times and dust formation is reduced, while theutilization of higher source flow fluxes allows faster growth rates tobe achieved. In a MEAGlow reactor higher film crystallinity than isobserved for RPECVD can be achieved as a result of the diffusion of thegroup III metal component on the substrate surface prior to the deliveryof the active nitrogen species.

Thus, the MEAGlow reactor can be used to reduce the formation of dustduring thin film growth by relatively high pressure film growthtechniques, for example, operating approximately over a range of 0.1mTorr to 10 Torr, compared to molecular beam epitaxy (MBE) whichoperates over a range of 0.000001 mTorr to 0.1 mTorr.

In various forms, modulating the supply of the anion species can be bychanging a chamber pressure of the plasma. The chamber pressure could beadjusted to optimize a flux of the anion species to the substrate regionand/or substrate while the plasma is on. Furthermore, the plasma canoperate in continuous or pulsed modes. The chamber pressure may beadjusted to optimize the flux of the cation species to the substrate orsubstrate region, and to potentially eliminate the need for a carriergas, so that cation species delivery could be by vapor phase deliveryalone.

It should be appreciated that modulating the supply of the cationspecies or the anion species to the substrate region, which includes theactual substrate, can be achieved by a change in the chamber pressurebetween the use of the anion species and the cation species, or theirrespective precursors.

The pressure of the chamber of the cation species, or cation speciesprecursor, can be relatively low to allow for delivery of themetalorganic without a carrier gas, i.e., the metalorganic can besupplied or delivered by vapor phase delivery alone when the chamberpressure is less than the vapor pressure of the cation species or cationspecies precursor. For supply or delivery of the anion species, thepressure, in a particular region such as for example a chamber housingan electrical source, can be optimized for efficient operation of ahollow cathode source, which operates in a narrow pressure rangedependent on the dimensions of the hollow cathode and the power applied.

In embodiments of the invention, a method for producing a Group IIImetal nitride thin film comprises alternately and sequentiallycontacting a substrate in a reaction chamber with a Group III metalprecursor and an active neutral species of nitrogen. In an embodiment,contacting the substrate with the Group III metal precursor forms anon-self limiting layer of a Group III metal over the substrate.

In an embodiment, the Group III metal precursor can include a vaporphase chemical comprising a Group III metal. In an embodiment, the GroupIII metal precursor comprises a metalorganic (or organometallic)species.

In an embodiment, the active neutral species of nitrogen comprisesnitrogen species having energies less than or equal to 7 eV. In anembodiment, the active neutral species of nitrogen comprises N₂ specieshaving the lowest excited state of molecular nitrogen (A³Σ_(u) ⁺, seeFIG. 6).

In embodiments, a method for forming a Group III metal nitride thin filmover a substrate comprises contacting the substrate with a Group IIImetal precursor for a first time period to form a layer of a Group IIImetal having a thickness greater than 1 monolayer (ML). Next the layerof the Group III metal is contacted with plasma-activated species ofnitrogen for a second time period to form a layer (or thin film) of aGroup III metal nitride.

In embodiments, the first time period is greater than or equal to 10seconds, or greater than or equal to 30 seconds, or greater than orequal to 1 minute, or greater than or equal to 10 minutes. In anembodiment, the layer or thin film of the Group III metal nitride has athickness greater than 1 ML, or greater than or equal to about 2 ML, orgreater than or equal to about 5 ML. In an embodiment, the layer or thinfilm of the Group III metal nitride has a thickness greater than orequal the thickness of a quantum well.

In an embodiment, the plasma-activated species of nitrogen comprisesnitrogen species having the lowest excited state of molecular nitrogen(A³Σ_(u) ⁺).

In embodiments, a method for forming a Group III metal nitride thin filmover a substrate in a reaction chamber comprises alternately andsequentially pulsing into the reaction chamber a Group III metalprecursor and plasma-activated species of nitrogen, with each pulse ofthe Group III metal precursor forming a non-self limiting layer of aGroup III metal over the substrate. In an embodiment, theplasma-activated species of nitrogen comprises nitrogen species havingthe lowest excited state of molecular nitrogen (A³Σ_(u) ⁺). In anembodiment, the non-self limiting layer of the Group III metal has athickness greater than 1 monolayer (ML). In another embodiment, thenon-self limiting layer of the Group III metal has a thickness greaterthan 2 ML. In an embodiment, plasma-activated species of nitrogen havingenergies greater than 7 eV are quenched with, or prior to, each pulse ofthe plasma-activated species of nitrogen.

In other embodiments, a method for forming a Group III metal nitridethin film on a substrate comprises supplying a metalorganic species intoa reaction chamber for a first time period to form a layer of a GroupIII metal having a thickness greater than 1 monolayer (ML). In anembodiment, the layer of the Group III metal is a non-self limitinglayer of a Group III metal. Next, the metalorganic species is evacuatedfrom the reaction chamber. In an embodiment, the metalorganic species isevacuated by directing N₂ into the reaction chamber. In anotherembodiment, evacuation can be achieved with the aid of a vacuum systemalone or in combination with the use of N₂. Next, plasma-activatedspecies of nitrogen are supplied into the reaction chamber for a secondtime period to form a layer of a Group III metal nitride. In anembodiment, the supply (or feed) of the metalorganic species isterminated before supplying the plasma-activated species of nitrogeninto the reaction chamber.

In various embodiments, plasma-activated species of nitrogen aresupplied to the reaction chamber by first forming the plasma-activatedspecies of nitrogen with the aid of a plasma generator, and directing asubset of the plasma-activate species of nitrogen to the reactionchamber. In an embodiment, plasma-activated species of nitrogen areformed by supplying nitrogen (N₂) gas into the plasma generator. Next,the plasma-activated species of N₂ is generated in the plasma generator.In an embodiment, this is achieved by supplying power to the plasmagenerator. In an embodiment, plasma-activated species of nitrogen havingpotential energies greater than about 7 eV are quenched andplasma-activated species of nitrogen having potential energies less thanor equal to about 7 eV are supplied in the reaction chamber. In anembodiment, the pressures in one or both of the plasma generator and anarea downstream of the plasma generator (such as, e.g., the pressure inthe reaction chamber) are selected such that plasma-activated species ofnitrogen having potential energies greater than about 7 eV are quenched.

In various embodiments, the plasma generator comprises a gasdistribution member for providing the plasma-activated species of N₂ tothe reaction chamber (see, e.g., FIG. 4). In an embodiment, the gasdistribution member comprises a plurality of holes in a showerheadconfiguration. In another embodiment, the gas distribution membercomprises one or more hollow cathodes. In an embodiment, the plasmagenerator comprises a hollow cathode configured to generate theplasma-activated species of nitrogen using an electrical source selectedform the group consisting of a radiofrequency (RF) source, a lowerfrequency source, and a direct current (DC) source. In an embodiment,the metalorganic species are supplied to the reaction chamber with theaid of a gas distribution member (such as, e.g., the showerhead of FIG.5) having a plurality of holes.

In embodiments, the layer of the Group III metal nitride has a thicknessgreater than 1 monolayer (“ML”), or greater than or equal to about 2 ML,or greater than or equal to about 5 ML. In an embodiment, the layer ofthe Group III metal nitride has a thickness greater than or equal thethickness of a quantum well.

In an embodiment, the plasma-activated species of nitrogen comprisesactive neutral nitrogen species having potential energies less then orequal to about 7 eV.

In embodiments, a method for forming a Group III metal nitride thin filmover a substrate comprises (a) pulsing one of a Group IIImetal-containing reactant and plasma-activated species of nitrogen intoa reaction chamber; (b) evacuating the reaction chamber; (c) pulsing theother of the Group III metal-containing reactant and plasma-activatedspecies of nitrogen into the reaction chamber; and (d) repeating steps(a)-(c) until a Group III metal nitride thin film of predeterminedthickness is formed. In an embodiment, each pulse of the Group IIImetal-containing reactant forms a non-self limiting layer of a Group IIImetal on or over the substrate. In various embodiments, each pulse ofthe Group III metal-containing reactant forms a layer of a Group IIImetal having a thickness greater than 1 monolayer (ML), or greater thanor equal to about 2 ML, or greater than or equal to about 5 ML.

In certain embodiments, the reaction chamber can be evacuated betweensteps (c) and (d). This reduces, if not eliminates, the gas phasereaction between the Group III metal-containing reactant andplasma-activated species of nitrogen, which advantageously reduces, ifnot eliminates, dust formation. In an embodiment, after the pulse of theGroup III metal-containing reactant, the reaction chamber can beevacuated prior to pulsing the plasma-activated species of nitrogen intothe reaction chamber. In another embodiment, after the pulse of theplasma-activated species of nitrogen, the reaction chamber is evacuatedprior to pulsing the Group III metal-containing reactant into thereaction chamber. In an embodiment, the reaction chamber can beevacuated with the aid of an inert gas, such as Ar, He, or N₂. Inanother embodiment, the reaction chamber can be evacuated with the aidof a vacuum pumping system, such as a turbomolecular (“turbo”) pumpbacked by a mechanical pump. In another embodiment, the reaction chambercan be evacuated with the aid of an inert gas and a vacuum pumpingsystem.

With reference to FIG. 2, a pulsing sequence (also “pulsing train”herein) for forming a Group III metal nitride thin film is illustrated.FIG. 2 shows an on/off pulsing sequence for a metalorganic species (top)and active neutral nitrogen species (bottom), with flow rates (arbitraryunits) shown on the ordinates and time shown on the abscissa. In apreferable embodiment, a pulse of the metalorganic species does notoverlap a pulse of the active neutral nitrogen (also “active nitrogen”herein) species. FIG. 2 also illustrates the growth mode during materialpulses, designated as “On/off MEAGlow growth mode”. The growth modewithout material pulsing has also been illustrated (horizontal lines).

With continued reference to FIG. 2, in a first step, a pulse of ametalorganic species (or other vapor phase metal precursor) is providedinto a reaction chamber (i.e., metalorganic species pulse turned ‘on’)having a substrate on which a Group III metal nitride thin film is to beformed. During the pulse of the metalorganic species, the flow rate ofactive nitrogen is terminated (i.e., active nitrogen is not directedinto the reaction chamber). In an embodiment, this includes terminatingthe supply of N₂ gas into a plasma generator in fluid communication withthe reaction chamber. The pulse of metalorganic species is provided fora time period and flow rate selected to form a Group III metal thin filmof predetermined (or desired) thickness. Next, the flow of themetalorganic species into the reaction chamber is terminated. In apreferable embodiment, the Group III metal thin film formed in the firststep is non-self limiting.

Next, the reaction chamber can be optionally evacuated with the aid ofan inert gas and/or a vacuum pumping system (see above).

Next, following termination of the flow of the metalorganic species intothe reaction chamber, in a second step, active neutral nitrogen ispulsed into the reaction chamber. During the second step, the substrateand metal thin film formed over the substrate are exposed to activenitrogen. In an embodiment, during the second step, at least a portionof the Group III metal thin film reacts with the active nitrogen to forma Group III metal nitride thin film.

Next, the reaction chamber can be evacuated with the aid of an inert gasand/or a vacuum pumping system (see above). In an embodiment, this canentail maintaining the flow of N₂ into the reaction chamber but notsupplying power to the plasma generator, thus precluding the formationof active neutral nitrogen species.

Next, the first step and the second step, in addition to the evacuationsteps, can be repeated until a Group III metal nitride layer or thinfilm of predetermined (or desired) thickness is formed over thesubstrate. Various process parameters, such as the duration of each ofthe metalorganic species and the active neutral nitrogen species pulses,chamber pressure, substrate temperatures and material fluxes, can beadjusted to achieve Group III metal nitride thin films with desiredqualities and within a predetermined amount of time.

In various embodiments, with the pulse of a metalorganic species and thepulse of active nitrogen defining a cycle, a thin film can be formedfollowing 2 or more cycles, or 5 or more cycles, or 10 or more cycles,or 20 or more cycles. It will be appreciated that a cycle can includeone or more evacuation steps between the metalorganic species pulse andactive nitrogen pulse.

In embodiment, the duration of a metalorganic species pulse can begreater than or equal to 10 seconds, or greater than or equal to 30seconds, or greater than or equal to 1 minute, or greater than or equalto 10 minutes. In embodiments, the duration of an active nitrogen pulsecan be greater than or equal to 10 seconds, or greater than or equal to30 seconds, or greater than or equal to 1 minute, or greater than orequal to 10 minutes.

By alternately and sequentially pulsing into a reaction chamber a GroupIII metal precursor and active neutral nitrogen species, improved growthrates and thin film properties (quality, device performance) can beachieved. Pulsing methods and systems of embodiments of the inventioncan advantageously provide for improved growth rates.

Plasma Processing Reactors

In an aspect of the invention, plasma processing reactors (also “plasmareactors” herein) are provided for forming thin films. In embodiments,plasma processing reactors comprise MEAGlow reactors. In embodiments,the plasma processing reactors can be used to form Group III metalnitride thin films or layers, such as, e.g., gallium nitride thin filmsand indium nitride thin films.

In embodiments of the invention, plasma processing reactors can be usedto form active neutral nitrogen species. In an embodiment, plasmaprocessing reactors can be used to form active neutral nitrogen specieshaving potential energies less than or equal to about 7 eV. In anembodiment, plasma processing reactors can be used to form activeneutral nitrogen species having the lowest excited state of molecularnitrogen (A³Σ_(u) ⁺, see FIG. 6). In a preferable embodiment, plasmaprocessing reactors are used to form active neutral nitrogen speciesfrom molecular nitrogen (N₂).

In an embodiment, a method for forming active neutral nitrogen species,comprises supplying nitrogen (N₂) gas into a plasma generator. Next,plasma-activated species of N₂ are generated in the plasma generator. Inan embodiment, plasma-activated species of N₂ includes nitrogenradicals, nitrogen cations and nitrogen anions. In another embodiment,plasma-activated species of N₂ includes active neutral species ofnitrogen. In a preferable embodiment, plasma-activated species of N₂having potential energies greater than about 7 eV are subsequentlyquenched. In embodiments, quenching of the higher energy species (e.g.,high energy plasma-activated species of N₂ having energies greater than7 eV) can be achieved by controlling the number of gas collisions thatsuch high energy species undergo. In various embodiments, the pressureor pressures in one or both of the plasma generator and an areadownstream of the plasma generator (such as, e.g., the pressure in areaction chamber downstream of the plasma generator) are selected suchthat plasma-activated species of nitrogen having potential energiesgreater than about 7 eV are quenched. The distance in which quenchingoccurs (i.e., the distance high energy species travel before beingquenched via collision with other gas phase species) is also dependenton the gas temperature and the flow rate of the gas, so that, in variousembodiments, the gas temperature, the gas flow rate and/or the pressurewill determine the distance that these species will travel before beingquenched. In another embodiment, distance itself can be used to quenchspecies with potential energy higher than 7 eV. In an embodiment,quenching can be achieved by selecting the distance between the plasmagenerator and the substrate in the reaction chamber. In anotherembodiment, the pressure in the plasma generator and/or downstream ofthe plasma generator selected to achieve such quenching (i.e., quenchingplasma-activated species of nitrogen having potential energies greaterthan about 7 eV) is between about 0.1 mTorr and 10 Torr. Next,plasma-activated species of N₂ having potential energies less than orequal to about 7 eV, are then supplied to a reaction chamber (also“reactor chamber” and “chamber” herein) having a substrate. Thesubstrate is then exposed to (or contacted with) such plasma-activatedspecies of N₂.

In an embodiment, a method for providing active neutral nitrogen (N₂) toa reaction chamber comprises supplying nitrogen (N₂) gas into a plasmagenerator. Next, a first group of plasma-activated species of N₂ isformed in the plasma generator. A second group of plasma-activatedspecies of N₂ is then formed from the first group, the second groupcomprising active neutral nitrogen species having potential energiesless than or equal to about 7 eV. The second group is then directed tothe reaction chamber having a substrate. In an embodiment, N₂ gas issupplied into the plasma generator at a pressure greater than or equalto about 0.1 mTorr. In another embodiment, N₂ gas is supplied into theplasma generator at a pressure greater than or equal to about 0.1 mTorrand less than or equal to 10 Torr.

In embodiments, a reactor for forming Group III metal nitride thin filmsor layers comprises a reaction chamber and a substrate holder disposedin the reaction chamber, the substrate holder configured to hold asubstrate. The reactor further comprises a plasma generator in fluidcommunication with a nitrogen (N₂), ammonia (NH₃), and/or hydrazine feedand the reaction chamber, the plasma generator configured to form activeneutral species of nitrogen. The reactor further comprises a controlsystem (or computer system) configured to alternately and sequentiallyprovide into the reaction chamber a Group III metal precursor and activeneural species of nitrogen. In an embodiment, the control system isconfigured to rotate a substrate on the substrate holder while thesubstrate is alternately and sequentially exposed to a Group III metalprecursor and active neutral species of nitrogen.

In an embodiment, the control system (such as control system 495 of FIG.4) is configured to control various process parameters, such as, forexample, substrate and/or substrate holder temperature, reactorpressure, reaction chamber pressure, plasma generator pressure, the flowrate of gas (e.g., N₂) into the plasma generator, the flow rate of gas(e.g., metalorganic species, active neutral species of nitrogen) intothe reaction chamber, the rate at which the substrate rotates duringthin film formation, power to the plasma generator (e.g., DC or RFgenerator), and a vacuum system in fluid communication with the reactionchamber. The vacuum system can comprise various pumps configured toprovide vacuum to the reaction chamber, such as, e.g., one or more of aturbomolecular (“turbo”) pump, a cryopump, an ion pump and a diffusion,in addition to a backing pump, such as a mechanical pump.

In various embodiments, the plasma generator comprises a gasdistribution member for providing active neutral species of nitrogen tothe reaction chamber. In an embodiment, the gas distribution membercomprises a plurality of holes in a showerhead configuration. In anembodiment, the gas distribution member comprises one or more hollowcathodes. In embodiments, the gas distribution member can be configuredto form a group of active neutral species of nitrogen having potentialenergies less than or equal to about 7 eV. In a preferable embodiment,the plasma generator is configured to filter active neutral species ofnitrogen having potential energies greater than about 7 eV to provideinto the reaction chamber active neutral species of nitrogen havingpotential energies less than or equal to about 7 eV. In embodiments,during plasma formation, active nitrogen having the lowest excited stateof molecular nitrogen (A³Σ_(u) ⁺, see FIG. 6) is formed from a vapor orgas, or a plasma mixture of a vapor or gas, comprising various plasmaexcited species of nitrogen, thereby providing to a reaction chamberactive nitrogen species having the lowest excited state of molecularnitrogen (A³Σ_(u) ⁺). In various embodiments, the pressure in the plasmagenerator and/or the reaction chamber is selected such that high-energyactive neutral species of N₂ (i.e., active neutral species of N₂ havingpotential energies higher than the lowest excited state of N₂), inaddition to other plasma-excited species of nitrogen, are quenched viagas phase collisions. In an embodiment, the plasma generator pressurefor providing active neutral species of nitrogen substantially havingthe lowest excited state of nitrogen (A³Σ_(u) ⁺) is between about 0.1mTorr and 10 Torr.

In embodiments, the reactor further comprises a Group III metalprecursor feed for directing a Group III metal precursor to the reactionchamber. In an embodiment, the Group III metal precursor feed isdisposed adjacent the substrate holder. In an embodiment, the Group IIImetal precursor feed is disposed between the substrate holder and theplasma generator. In some embodiment, the Group III metal precursor feedcovers a portion of the substrate holder.

In other embodiments, the Group III metal precursor feed comprises ahollow head having a narrow end and a wide end, the narrow endconfigured to be positioned above a central portion of a substrate onthe substrate holder, the wide end configured to be positioned above anouter portion of the substrate. In such a case, in an embodiment, asurface of the hollow head facing the substrate holder is provided witha plurality of holes configured to provide a Group III metal precursorto at least a portion of a substrate in the reaction chamber.

In various embodiment, upon supplying power to the plasma generator(e.g., RF generator), plasma excited species of nitrogen, such as activeneutral species of nitrogen having various potential energies, nitrogenanions, nitrogen cations, and nitrogen radicals, are formed. Next,active neutral species of nitrogen having potential energies greaterthan about 7 eV are quenched, and active neutral species of nitrogenhaving potential energies less than or equal to about 7 eV are providedinto the reaction chamber. In an embodiment, such species are quenchedvia collision with other gas phase species, the walls of the plasmagenerator, and/or the walls of the reaction chamber. In an embodiment,with the plasma generator pressure selected to be between about 0.1mTorr and 10 Torr, active neutral species of nitrogen having potentialenergies greater than about 7 eV, in addition to other plasma excitedspecies of nitrogen, are quenched, and active neutral species ofnitrogen having potential energies less than or equal to about 7 eV areprovided for distribution to the reaction chamber having the substrate.In an embodiment, the pressure in the plasma generator is adjusted viathe flow rate of a nitrogen-containing species into the plasmagenerator. In an embodiment, the pressure in the reaction chamber can bethe same or nearly the same as the pressure in the plasma generator,such that a change in reaction chamber pressure effects a change in theplasma generator pressure, and vice versa.

With reference to FIG. 3, there is illustrated a schematic of an exampleplasma processing reactor that provides a MEAGlow reactor 200. Mainchamber 210 is where reactions between chemical species occurs. A plasmapower source 215 creates a contained plasma where plasma species aresupplied by plasma supply lines 220. Plasma power source 215 can becooled by water inlet/outlet 225. Main chamber 210 contains a substratethat can be adjusted in height by pneumatic sample lift 230. The regionbelow the substrate can be connected to pump line 235 to assist increating a vacuum in main chamber 210. Main chamber 210 is connected toload lock 240 via gate valve 245. Main chamber 210 is also connected toconflat cross 250 via gate valve 255.

An optical omission spectrometer optical fibre (or fiber) 260 can beintroduced into main chamber 210 for diagnostic purposes. A furtherwater inlet/outlet 265 and a purge valve 270 are associated with mainchamber 210. A metaloraganic inlet 275 supplies a metalorganic speciesto main chamber 210. A bypass pump 280 is also connected to metalorganicinlet line 275.

Load lock 240 is connected to dry pump 285 with associated waterinlet/outlet 290. A transfer arm 295 is associated with load lock 240.Wide range gauge 300 can be used to measure the pressure on dry pump 285side of load lock 240. Throttle valve 305 and filter 310 connect pumpline 235 to dry pump 285.

Conflat cross 250 is connected to a turbo pump 315 which is connected toa backing pump 320 via filter 325 and electrical isolation valve 330.Backing pump 320 and dry pump 285 exhaust gas into exhaust line 335. RGA340 is connected to conflat cross 250 which can also be provided with anassociated wide range gauge 345.

With reference to FIG. 4, there is illustrated a schematic of an examplemain chamber of a plasma processing reactor providing part of a MEAGlowreactor. Main chamber 400 includes housing 405 enabling a vacuum to becreated in main chamber 400. Gas flow outlets 410 and 415 are connectedto a vacuum system to remove gases from main chamber 400. Metalorganicspecies 430 is introduced internally into region 460 of main chamber 400via metalorganic inlet 420, which is connected to “shower head” feed425. Shower head (also “showerhead” herein) feed 425 directsmetalorganic species 430 onto a substrate, which is held by or placed onsubstrate holder 435. Substrate holder 435 is supported by, and can beheated by, pedestal 440.

A hollow cathode 445 is provided above a ground grid 450. Gas flowsthrough hollow cathode 445 and ground grid 450 from plasma creationregion 455 into reaction region 460, being in the vicinity of thesubstrate on substrate holder 435.

The hollow cathode 445 can generate a plasma using a variety ofelectrical sources, for example a radiofrequency (“RF”) source, a lowerfrequency source, a higher frequency source, and/or a DC source. This atleast partially enables the plasma to be scalable to a relatively largearea.

Anode 465 is supported by insulator supports 470 and attached to powerline 475. A plasma based species is introduced into plasma creationregion 455 via plasma gas inlet 480. A plasma can thus be created inregion 455 that diffuses into region 460 to react with metalorganicspecies 430 on the substrate.

A standard capacitively coupled plasma can be formed between anode 465and hollow cathode 445. This can be achieved by RF excitation of theanode 465 from RF power supply line 475. In this case, the plasma itselfcan act as a virtual anode, or by DC excitation of anode 465. There issome evidence to suggest that DC excitation results in higher densityplasmas. See P. Virostko, Z. Hubicka, P. Adamek, M. Cada, J. Olejnicek,M. Tichy and M. Sicha Contrib. Plasma Phys. 46 (2006) 445-450, which isentirely incorporated herein by reference. In the holes in cathode 445,at certain gas flows and pressures, dependent on the geometry of theholes, a very strong additional plasma can be achieved due to the hollowcathode effect. Any additional plasma created by the hollow cathodeeffect is contained well above the substrate/sample by ground grid 450,since energetic ions can be damaging to the thin film during filmgrowth.

Various advantages can be achieved using the MEAGlow arrangement. Forexample, the growth rate of a material at the substrate can be increasedcompared to normal RPECVD growth. Also, plasma power sources other thanmicrowave sources can be effectively applied. In particular, RF (RadioFrequency) and other lower frequency sources including DC can be used,particularly in the example case of a hollow cathode source.

For MEAGlow, a hollow cathode source can be used under DC conditions andthis can be advantageous in terms of obtaining an improved flux ofactive neutrals. In this particular but non-limiting example, because amicrowave source is not used, relatively very large area deposition canbe achieved.

In another example embodiment, oxygen contamination can be reduced byusing a capacitively coupled parallel plate configuration, to eliminatethe oxygen contamination that occurs when a plasma is in contact withdielectric windows. The parallel plate configuration can be used inconjunction with a hollow cathode plasma source to initiate a higherdensity plasma.

With continued reference to FIG. 4, the plasma processing reactorfurther comprises a control system 495 for controlling various processparameters and components, such as, for example, the substrate and/orsubstrate holder temperature, main chamber pressure, the flow rate ofplasma gas (e.g., active neutral species of N₂), the flow rate of ametalorganic species, the rate at which the substrate rotates duringthin film formation, power to the plasma generator (e.g., RF generator),and a vacuum system in fluid communication with the reaction chamber.The control system 495 can further control the pressure of the mainchamber 400 and the pressure of the plasma creation region 455. In anembodiment, the control system is configured to alternately andsequentially provide non-overlapping pulses of a metalorganic species(or other Group III metal precursor) and active neutral species ofnitrogen into the main chamber during thin film formation.

With reference to FIG. 5, there is illustrated a schematic of an example“shower head” (also “showerhead” herein) feed 425 that can be used inMEAGlow reactor chamber 400. Shower head feed 425 distributesmetalorganic species to the substrate/sample. The shower head feed 425can be located relatively close to the substrate/sample holder 435 ascompared to a normal RPECVD system configuration. A preferred holedispersion pattern for the shower head feed 425 is illustrated in FIG.4. Holes 428 are formed in a surface of the shower head feed 425 torelease metalorganic species. Holes 428 direct metalorganic species 430onto the substrate/sample. For the purposes of illustration only, asample holder recess 485 and a sample holder lip 490, being part ofsubstrate holder 435, are illustrated in relative position to showerhead feed 425 and holes 428. A typical sample/substrate would be placedwithin sample holder recess 485. Preferably, substrate holder 435 isrotated about a longitudinal axis so that a substrate/sample rotatesunder shower head feed 425 and so that metalorganic species is evenlydistributed on the substrate/sample.

According to other examples, the metalorganic delivery head can be ofvarious configurations. The metalorganic delivery head may besubstantially circular and cover the substrate, with exit holes providedabout the circular extent of the head (i.e., a ‘showerhead’configuration). This configuration might require the head to be moved(e.g., moved laterally) between pulses of metalorganic species. Themetalorganic delivery head may be of a ring-like or annularconfiguration with exit holes positioned about the ring-like or annulargeometry. The metalorganic delivery head may be of a form where themetalorganic species is introduced via a central hole or duct and isdispersed over the substrate by a dispersing mechanism, such as arotating component creating a centrifugal dispersing action (e.g., a“turbodisc” configuration).

For the deposition of group III nitride semiconductor thin films byordinary known RPECVD methods, the inventor has found that RF generatedplasma supplies operating at 13.56 MHz have not proven particularlyeffective, with too much dust production being evident. In contrast,2.45 GHz microwave plasma systems have proven to be more effective withsubstantially less dust production. It has been reported that formicrowave generated plasmas less energy is required to sustain anelectron-ion pair. For argon plasmas it has been estimated that 2-7times less power per electron-ion pair is required at 2.45 GHz than atRF frequencies—dependent upon the discharge conditions. See M. Moisan,C. Barbeau, R. Claude, C. M. Ferreira, J. Margot, J. Paraszczak, A. B.Sa, G. Sauve and M. R. Wertheimer J. Vac. Sci. Technol. B 9 (1991) 8,which is entirely incorporated herein by reference. Hence, there isexpected a greater degree of ionisation in a microwave generated plasmacompared to an RF generated plasma for a given applied power. The excessenergy used to generate an electron-ion pair for the RF case eventuallydevolves to heat, which would promote gas phase reactions and theformation of dust during ordinary RPECVD film growth. However, aspointed out by Lieberman and Boswell the electron density (and hence thedegree of ionization—or electron-ion density) of an RF generated plasmais highly dependent on the means of generation. See M. A. Lieberman andR. W. Boswell, J. Phys. IV France 8 (1998) Pr7-145, which is entirelyincorporated herein by reference. Capacitively coupled RF plasmageneration (commonly used for semiconductor processing) is the leasteffective means, with electron-ion densities typically around 10⁹ to10¹⁰ cm⁻³. See M. A. Lieberman and R. W. Boswell, J. Phys. IV France 8(1998) Pr7-145, which is entirely incorporated herein by reference.While inductively coupled RF plasmas can typically have densities of10¹¹ to 10¹² cm⁻³. See M. A. Lieberman and R. W. Boswell, J. Phys. IVFrance 8 (1998) Pr7-145, which is entirely incorporated herein byreference. This is similar to the densities achieved by microwave plasmasystems (see, e.g., H. Tahara, K. Minami, T. Yasui, K. Onoe, Y.Tsubakishita and T. Yoshikawa, Jpn. J. Appl. Phys. 32 (1993) 1822, whichis entirely incorporated herein by reference), though typically lesspower is used in the case of the microwave source to achieve suchdensities (see M. Moisan, C. Barbeau, R. Claude, C. M. Ferreira, J.Margot, J. Paraszczak, A. B. Sa, G. Sauve and M. R. Wertheimer J. Vac.Sci. Technol. B 9 (1991) 8, which is entirely incorporated herein byreference). Other types of RF, or lower frequency plasmas, which utilizeresonance characteristics, can be even denser. For instance, RF, andlower frequency, hollow cathode plasma sources, can produce highdensities of ion-electron pairs. See, e.g., P. Virostko, Z. Hubicka, P.Adamek, M. Cada, J. Olejnicek, M. Tichu and M. Sicha, Contrib. PlasmaPhys. 46 (2006) 445, which is entirely incorporated herein by reference.

For PECVD based processes, where substrates are in direct contact withthe plasma, a high level of ionic species is usually a positive forplasma processes. This is also the case for RPECVD, and it is importantto note that although the active species used in RPECVD film growth isnot the ionic species, a greater degree of ionization within the plasmawill generally translate into a denser (or more dense) concentration ofneutral species in the afterglow region. See S. M. Rossnagel, “GlowDischarge Plasmas and sources for Etching and Deposition”, Thin FilmProcesses II, p. 37-38, edited by J. L. Vossen and W. Kern (AcademicPress, San Diego, 1991), which is entirely incorporated herein byreference. Some RF based plasma systems may be suitable for RPECVD, ifheating of the metalorganic reactants in the gas phase by the plasmasource can be avoided.

In the case of RPECVD using a nitrogen gas source for the plasma, thelowest excited state of molecular nitrogen has an extremely longradiative lifetime, estimated to be as high as 2 seconds by some groups(See P. Choi, Y. Kaufman and R. Aliaga, Appl. Phys. Lett. 57 (1990) 440,which is entirely incorporated herein by reference) and is a majorcontributing species to nitride film growth by RPECVD. For a hollowcathode source this lowest excited molecular nitrogen state has beenobserved to be present at densities as high as about 4.9×10¹¹ cm⁻³. SeeP. Choi, Y. Kaufman and R. Aliaga, Appl. Phys. Lett. 57 (1990) 440,which is entirely incorporated herein by reference. In conventionalRPECVD, however, it is known that collisions with some impurity species,including CH₄, is gas kinetic and will rapidly quench this form ofneutral nitrogen at a rate of up to about 1000 times higher thancollisions with molecular nitrogen. See L. G. Piper, J. Chem. Phys. 87(1987) 1625, which is entirely incorporated herein by reference.

For RPECVD film growth where excited nitrogen molecular neutrals andmetalorganic species are present at the same time in the growth system,a notable reduction in the active nitrogen that reaches the substratecan be expected due to quenching caused by collisions with these methylgroup species, resulting in a lower then expected growth rate. Gas phasereactions due to the interaction of the metalorganic with the activeneutral nitrogen can also be expected. The inventor has observed astrong secondary light emission (chemiluminescence) in the fardownstream afterglow of a microwave generated nitrogen plasma whenmetalorganic is introduced into the system, which suggests that such gasphase interactions are in progress. Using a migration enhancedconfiguration, where the metalorganic is not introduced at the same timeas the active nitrogen should therefore allow a greater proportion ofactive species to reach the substrate to participate in film growth.

Microwave based plasma generation systems are electrodeless, a strongelectromagnetic field in a resonant cavity leads to gas breakdown. Adielectric window is used to transmit the electromagnetic field into thegas system where the plasma is generated, usually at low pressure. Ithas been found that plasma interaction with the dielectric window cancause contamination of the plasma by species being etched from thewindow. A lengthy surface passivation cycle, taking as long as two days,in a well evacuated vacuum system that has no exposure to atmosphere isneeded to create a nitride layer on the window to overcome this problem,as is outlined in International Patent Publication WO2006/034540, whichis entirely incorporated herein by reference. Because of the relativelyshort wavelengths of microwave sources and the need to have dimensionalcavities to sustain the plasma, it is also quite difficult to scalemicrowave sources for film deposition over large areas.

Although there are some advantages of the use of microwave plasmasources, the use of other sources, such as hollow cathode plasmasources, should allow for easier plasma source scalability and forreduced concern about contamination from windows. The use of a migrationenhanced growth scenario would allow other plasma sources to be usedwithout concern for gas heating which can result in enhanced dustformation problems. In particular, hollow cathode sources, which do notemploy dielectric windows could be used.

Another advantage of a microwave plasma generation system is the abilityto sustain the plasma over a very wide range of pressure. The inventorhas been able to sustain a microwave generated nitrogen plasma over a 22Torr to 10 mTorr range using a system capable of deliveringapproximately 600 W of power. Other RF and lower frequency (e.g., DC)generated plasmas do not generally have such a broad range of operatingpressure. Again, using a migration enhanced methodology allows separateconditions to be used for the application of the metalorganic and theplasma, so that the chamber pressure for the delivery of the activenitrogen can be tailored to the plasma source used. To prevent highenergy neutral species (such as atomic nitrogen) from reaching thesubstrate (which can happen at too low a growth pressure) the flow ratefrom the plasma source can be reduced and the distance from the plasmato the substrate can be adjusted, instead of adjusting the chamberpressure. This can provide a balance between having a high density oflow energy active neutral species for film growth, while minimizing thepresence of higher energy damaging species, which can affect filmquality and reduce the growth rate through etching.

The delivery of the metalorganic for a migration enhanced film growthregime can be optimized to enable a higher delivery rate to thesubstrate. The gas head for the metalorganic can be positioned quiteclose to the substrate holder, and relatively low delivery pressures canbe used to increase the utilization of the metalorganic. The requirementfor uniform radial and axial delivery in the chamber, necessary duringconventional RPECVD film growth, can be relaxed for film growth in aMEAGlow reactor, with only radial uniformity being a necessary conditionfor design of the metalorganic delivery head.

During normal RPECVD film growth, rotation is used to “smooth out” smallnon-uniformities that occur axially, but because film growth iscontinuous during the process, uniform conditions are required tomaintain uniform film properties that would otherwise be grown into thefilm. In contrast, for a MEAGlow reactor, the film growth only occursduring the application of the plasma. Metalorganic delivery cantherefore occur along a radius of the substrate holder so long asrotation of the substrate under that radius is rapid enough to provideuniform coverage of the substrate by the metal while the plasma sourceis off. The configuration of the metalorganic vapor delivery head cantherefore be greatly simplified. Continued rotation while the plasma ison and the metalorganic is off ensures that any shadowing by thedelivery head is not detrimental in terms of ensuring migration enhancedepitaxy occurs, and a uniform layer is deposited over the plasma onperiod.

With reference to FIG. 6, an energy diagram showing various activenitrogen species and their respective potential energies is shown, inaccordance with an embodiment of the invention. The lowest excited stateof molecular nitrogen, A³Σ_(u) ⁺, has a potential energy between about 6eV and 7 eV. The plot on the right shows the kinetic barrier for theformation of gallium nitride (GaN) from gallium and N₂(g). The change infree energy upon formation of GaN is about 1.9 eV.

With reference to FIG. 7A, there is shown by way of example only animage of an instance of migration enhanced type growth for an InN filmgrown in a RPECVD type reactor. The substrate is shown at the bottom ofthe figure, above which is the largely polycrystalline film, shown atroughly a 30 degree angle to the horizontal with columnar growth typicalof a hexagonal crystal structure being evident. At the top surfaceregion of the film, however, the columnar crystals are terminated withan apparently single crystal layer where lateral film growth has beengreatly enhanced. In contrast, with reference to FIG. 7B, there is shownby way of example only an image of an instance of non-migration enhancedtype growth for an InN film grown in a RPECVD type reactor. There is noevidence of a top laterally grown layer.

Methods and plasma reactor devices of embodiments of the invention,including MEAGlow systems, are capable of achieving an increased rate ofthin film growth while reducing, if not eliminating, dust formationproblems compared to conventional PECVD or RPECVD systems and methods. Areduction of dust formation problems and improved crystallinity due toimproved lateral growth can be expected in MEAGlow compared toconventional RPECVD. The film growth rate improvement for MEAGlow canoccur because of the use of higher fluxes of the precursor materials,without dust formation problems. Growth rate improvement for MEAGlow isalso expected because the active neutral plasma species is not consumedby impurities in the gas phase, in particular by a metalorganicprecursor material, i.e., more of the active species make it to thesubstrate region for a given flux of these active species.

For MEAGlow, plasma sources other than RF or microwave plasma sourcescan be used because gas heating by the plasma source is less of an issue(powder production is reduced regardless). Because the conditions fordelivery of a metalorganic cation and a plasma-generated anion are notcongruent the conditions for the delivery of each precursor can beindependently optimized. This also enables a simplified metalorganicdelivery head to be used for MEAGlow compared to RPECVD, or MOCVD. Thechamber pressure during the delivery of the metalorganic cation can alsobe greatly reduced so that the use of a carrier gas with themetalorganic (as is typically used for RPECVD, MOCVD and PECVD) is notnecessary. The metalorganic can be delivered as a pure vapor using amuch simplified gas delivery system for which carrier mixing with themetalorganic is not required.

Example

A sapphire substrate was provided in a reaction chamber (such as thereaction chamber of FIG. 4) of a plasma processing reactor. The plasmaprocessing reactor included a plasma generator for forming activeneutral species of nitrogen. Trimethylgallium was supplied to thereaction chamber at a pressure of about 0.1 Torr for a time period ofabout 1 minute. The substrate was maintained at a temperature of about500° C., a temperature above the decomposition temperature oftrimethylgallium. Under such conditions, gallium metal deposited on thesurface of the sapphire substrate via the decomposition of thetrimethylgallium. The flow rate of nitrogen was about 50 standard cubiccentimeters (“sccm”) from the plasma source during the deposition ofgallium. No power was provided to the plasma generator (i.e., no activeneutral species of nitrogen were formed) during the deposition ofgallium on the sapphire surface.

After about 1 minute, the flow of trimethylgallium into the reactionchamber was terminated. Next, the nitrogen flow through the plasmasource (or plasma generator) was increased to about 500 sccm. Thepressure in the chamber was increased to 3 Torr and plasma was struckwith a DC source operating at 200 W to generate active neutral speciesof nitrogen, which was directed at the substrate in the reactionchamber. The sapphire substrate (and the gallium thin film on thesapphire substrate) was exposed to active neutral species of nitrogenfor about 1 minute, at which point the DC source was turned off and thenitrogen flow rate was reduced to about 50 sccm. The active neutralspecies of nitrogen saturated the gallium on the substrate andprecipitated a GaN thin film (i.e., the Ga thin film was converted to aGaN thin film). The steps of alternately and sequentially directing intothe reactor trimethylgallium and active neutral species of nitrogen wererepeated 30 times to form a gallium nitride thin film having a thicknessof about 1000 nanometers.

Optional embodiments of the present invention may also be said tobroadly consist in the parts, elements and features referred to orindicated herein, individually or collectively, in any or allcombinations of two or more of the parts, elements or features, andwherein specific integers are mentioned herein which have knownequivalents in the art to which the invention relates, such knownequivalents are deemed to be incorporated herein as if individually setforth.

Although various embodiments have been described in detail, it should beunderstood that various changes, substitutions, and alterations can bemade by one of ordinary skill in the art without departing from thescope of the present invention.

It will be appreciated that methods, apparatuses and systems ofembodiments of the invention can be used to form metal oxides. In such acase, plasma activated species of oxygen can be supplied to the reactionchamber having a substrate.

It should be understood from the foregoing that, while particularimplementations have been illustrated and described, variousmodifications can be made thereto and are contemplated herein. It isalso not intended that the invention be limited by the specific examplesprovided within the specification. While the invention has beendescribed with reference to the aforementioned specification, thedescriptions and illustrations of the preferable embodiments herein arenot meant to be construed in a limiting sense. Furthermore, it shall beunderstood that all aspects of the invention are not limited to thespecific depictions, configurations or relative proportions set forthherein which depend upon a variety of conditions and variables. Variousmodifications in form and detail of the embodiments of the inventionwill be apparent to a person skilled in the art. It is thereforecontemplated that the invention shall also cover any such modifications,variations and equivalents.

What is claimed is:
 1. A method for producing a Group III metal nitridethin film using plasma enhanced chemical vapor deposition, comprisingalternately and sequentially contacting a substrate in a reactionchamber with a Group III metal precursor and an active neutral speciesof nitrogen, wherein the active neutral species of nitrogen is suppliedas a plasma, and wherein a supply of the Group III metal precursor and asupply of the active neutral species of nitrogen to the substrate areintermittently modulated by: pulsing the supply of the active neutralspecies of nitrogen by changing a chamber pressure of the plasma whilethe plasma is on; and separately pulsing the supply, by vapor phasedelivery, of the Group III metal precursor.
 2. The method of claim 1,wherein contacting the substrate with the Group III metal precursorforms a non-self limiting layer of a Group III metal over the substrate.3. The method of claim 1, wherein the active neutral species of nitrogencomprises nitrogen species having the lowest excited state of molecularnitrogen (A³Σ_(u) ⁺) and are produced at pressures greater than 0.1mTorr using a hollow cathode plasma source.
 4. The method of claim 1,wherein the Group III metal precursor comprises a metal organic species.5. The method of claim 1, further including waiting for a period of timebetween the supply of the Group III metal precursor and the supply ofthe active neutral species of nitrogen to the substrate.
 6. The methodof claim 1, wherein changing the chamber pressure of the plasma whilethe plasma is on or pulsing the supply by vapor phase delivery of theGroup III metal precursor is by reducing or stopping a gas flow at aninlet.
 7. The method of claim 1, wherein the supply of the Group IIImetal precursor and the supply of the active neutral species of nitrogento the substrate are repeatedly intermittently modulated until a desiredthickness of the thin film is obtained.
 8. The method of claim 1,further including: supplying the Group III metal precursor to thesubstrate when there is at most a relatively low flux of the activeneutral species of nitrogen near the substrate; and supplying the activeneutral species of nitrogen to the substrate when there is at most arelatively low flux of the Group III metal precursor near the substrate.9. The method of claim 1, wherein the plasma has a plasma pressure rangeof 0.1 mTorr to 10 Torr.
 10. The method of claim 1, wherein the chamberpressure of the plasma is changed to optimize a flux of the activeneutral species of nitrogen to the substrate while the plasma is on. 11.A method for forming a Group III metal nitride thin film over asubstrate in a reaction chamber using plasma enhanced chemical vapordeposition, comprising alternately and sequentially pulsing into thereaction chamber a Group III metal precursor and plasma-activatedspecies of nitrogen, each pulse of the Group III metal precursor forminga non-self limiting layer of a Group III metal over the substrate,wherein the plasma-activated species of nitrogen is supplied as aplasma, and wherein a supply of the Group III metal precursor and asupply of the plasma-activated species of nitrogen to the substrate areintermittently modulated by: pulsing the supply of the plasma-activatedspecies of nitrogen by changing a chamber pressure of the plasma whilethe plasma is on; and separately pulsing the supply, by vapor phasedelivery, of the Group III metal precursor.
 12. The method of claim 11,wherein the plasma-activated species of nitrogen comprises nitrogenspecies having the lowest excited state of molecular nitrogen (A³Σ_(u)⁺) and are produced at pressures greater than 0.1 mTorr using a hollowcathode plasma source.
 13. The method of claim 11, wherein the non-selflimiting layer of the Group III metal has a thickness greater than 1monolayer (ML).
 14. The method of claim 11, wherein plasma-activatedspecies of nitrogen having energies greater than 7 eV are quenched bygas collision before reaching the thin film.